An example of an active matrix display panel as defined above is known, e.g., from U.S. Pat. No. 5,903,246. In the known panel, a circuit is coupled to a current source for driving a plurality of active organic light emitting diodes (OLEDs) arranged in a column at a desired brightness. The circuit comprises an input leg of a current mirror for establishing a reference current for driving an active OLED, a plurality of selecting means, responsive to a row select signal, for respectively selecting an OLED from the plurality of active OLEDs; an output leg of a current mirror for supplying a mirror of the established reference current to the selected OLED; and a plurality of charging means for supplying a mirror of the established reference current in order to continuously drive the selected O-LED. The known technique includes separate, digitally adjustable current sources on each column line of the array. For each column, the digitally-programmed current flow terminates with a reference OLED and a series transistor forming the input leg of a distributed current mirror. A reference current is used to establish a proper current by way of distributed current mirror circuitry driving any one of the active O-LED pixels in a column. In particular, a column select conductor, which is coupled to a digitally-programmable current source, supplies current to a transistor and to a reference pixel, both coupled to the last pixel in the column.
In the known circuit, as the panel is scaled up, parasitic capacitance, which increases with the length of the column select conductor, provides a limit to the speed at which the digital current source can vary the new current level for each consecutively selected pixel in the column. In particular, large current swings cannot be accurately imposed within the time available for addressing a row. In addition, it becomes more difficult to match the transistors in the distributed current mirror as they lie further apart, so further decreasing the accuracy with which the current level determining the intensity of light emitted by the light-emitting element can be set within the time available for addressing a row.